Aperson with blood group a produces antibodies against b. a person with blood group b produces antibodies against a. a person with blood group ab produces no antibodies. how can you explain this? a and b alleles are co-dominant. therefore, both a and b carbohydrates are present on the cell. a is incompletely dominant over b. both a and b carbohydrates are present on the cell. a and b alleles are co-dominant. half the cells have only a, and other half have only b.